For operation of a semiconductor chip, it is installed in a lead frame having leads to connect the chip via its lead frame terminals (or external pins) to external circuits. Prior to installation and/or after installation in the lead frame, a microprocessor chip is tested as to whether the microprocessor chip operates normally and meets the specification of a product for which the microprocessor chip is used. That is, a plurality of test modes are required when different test conditions are needed to test whether, for example, a CPU, a RAM or peripheral devices of the microprocessor operates normally, when a program stored in an internal or external ROM is normally performed, or when read/write operations are normally performed.
In fabricating a semiconductor integrated circuit chips, it is difficult for a manufacturer to allocate a plurality of test pins because as the number of test pins increases, manufacture cost increases accordingly. Moreover, even when a plurality of test pins are allocated, they become useless after the tests are performed. Thus, the manufacturer tries to reduce the number of test pins and thus limits the number of possible test modes. In general, a single pin is allocated for a test, and a test mode is set by applying a logic high signal or a logic low signal.
In addition to being constrained by such limited test modes, the prior art has another disadvantage that it is influenced by process parameters because a test mode is set by changing a high level.